1. Field of the Invention
The present invention relates to a method and an apparatus for processing a digital signal and, more particularly, to a method and an apparatus adapted for use in recording, reproducing and transmitting an audio signal or the like with a high quality.
2. Description of the Prior Art
In the digital signal processing apparatus of the type mentioned, there are known some conventional examples employing the technique of adaptive predictive coding (APC) for transmission of an audio signal so as to achieve a high efficiency in the transmission while preventing deterioration of the signal-to-noise ratio and the articulation, as disclosed in Japanese Patent Laid-open Nos. 59-223033, 60-223034, 61-158217 and 61-158218.
In feedforward type and feedback type digital signal processing apparatus designed for transmitting an input digital signal by the use of such adaptive predictive coding, the input digital signal is requantized by the technique of linear predictive coding (LPC).
When the signal is requantized on the transmitting side in the digital signal processing apparatus of the type mentioned above, it is impossible to avert generation of some noise (hereinafter referred to as requantization noise). For the purpose of solving this problem, there are proposed contrivances to improve the aural signal-to-quantization noise ratio (SNR) by applying the technique of noise shaping. (IEEE Transaction on Acoustics, Speech, and Signal Processing, Vol. ASSP-27, No. 3, June 1979; Journal of Electronic Data Communication Society, Apr. 1987, Vol. 70 No. 4, pp. 392-400; Japanese Patent Laid-open Nos. 59-223032, 60-103746 and 61-158220).
In the feedback type digital signal processing apparatus where merely a single predictive filter is employed, the entire constitution can be simplified as compared with the feedforward type digital signal processing apparatus.
Therefore, similarly to the case of applying the noise shaping means to the feedforward type digital signal processing apparatus, application of such noise shaping means to the feedback type digital signal processing apparatus is considered effective to simplify the apparatus constitution as a whole.
However, in applying the noise shaping means to the feedback type digital signal processing apparatus, there occurs the necessity of executing complicated selective switchover of the noise filter characteristic simultaneously with the predictive filter in conformity to the input digital signal, hence raising another problem that renders the whole constitution intricate and prolongs the required processing time.
Furthermore, in both feedforward type and feedback type, it has been impossible heretofore to attain sufficient improvements by merely adjusting the noise shaping characteristic to be coincident with the input signal frequency characteristic.
Since the masking effect has unsymmetrical characteristic to the frequency, satisfactory effect is achievable in case the noise frequency is higher than the audio signal frequency. In contrast therewith, when the noise frequency is lower than the audio signal frequency, the masking effect is diminished to consequently bring about a problem of causing deterioration in the aural signal-to-quantization noise ratio.